Reversibly crosslinked silicones by Lewis Acid-Lewis Base pair formation / Author Patrick Rettenwander, Bsc.

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JOHANNES KEPLER UNIVERSITY LINZ Altenberger Straße 69 4040 Linz, Austria jku.at DVR 0093696 Author Patrick Rettenwander, Bsc. Submission School of Education Thesis Supervisor Assoc. Univ.-Prof. Dr. Uwe Monkowius Assistant Thesis Supervisor

Assoc. Univ.-Prof. Dr. Ian Teasdale December 2019

REVERSIBLY

CROSSLINKED

SILICONES BY

LEWIS-ACID-LEWIS-BASE PAIR

FORMATION

Master’s Thesis

to confer the academic degree of

Diplom-Ingenieur

in the Master’s Program

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December 2019 Patrick Rettenwander 2/83

SWORN DECLARATION

I hereby declare under oath that the submitted Master’s Thesis has been written solely by me without any third-party assistance, information other than provided sources or aids have not been used and those used have been fully documented. Sources for literal, paraphrased and cited quotes have been accurately credited.

The submitted document here present is identical to the electronically submitted text document.

Linz, 03.12.2019

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ACKNOWLEDGEMENT

First of all, I would like to thank my supervisors Assoc. Univ. Prof. Dr. Ian Teasdale and Assoc. Univ.-Prof. Dr. Uwe Monkowius for giving me the opportunity to do this thesis and constantly aiding me with professional assistance and ideas during the research progress. I would also like to express my gratitude to Univ. Prof. Dr. Oliver Brüggemann for enabling me the work at the Institute of Polymer Chemistry.

Furthermore, I would like to thank everyone of the ICP team for the great collaboration and the reliable help at any time difficulties occurred and the great atmosphere during my time at the institute. At his point I especially want to mention Kitti Rakos, BSc., DI Florian Gruber and Andreas Schnölzer for their support. Furthermore, I want to express my deepest appreciations to my office mates DI Michael Kneidinger and Christine Fiedler, BSc. for the great conversations.

I would also like to offer my special thanks do Dr. Matthias Bechmann for his help in NMR spectroscopy experiments, Assoc. Univ.-Prof. Dr. Mario Waser for his advices concerning Grignard reactions as well as DI Michael Winter and Farzad Abdallah, BSc. for recording ESI MS spectroscopic data of my compounds.

My deepest appreciation also goes to my fellow students DI Julia Felicitas Schwarz, DI Jessica Michalke, DI Magdalena Muhr, Kurt Wernecke, BSc., DI Gregor Hacker, Felix Mayr, BSc., and Daniel Köprunner, BSc. who got really close friends of mine during my studies. I especially want to mention Kurt who always motivated me during the whole time at the university, Magdalena for the many social activities when I was in Linz and Jessica for the countless deep conversations. Additionally, I owe a very important debt to my lab mate Julia for the great collaboration that made many practical courses a lot easier.

I would also like to express my gratitude to my childhood friends Stephan Steinherr, Florian Pachnek, BEng., Markus Seidl, Thomas Seidl, and Christopher Seidl for the great moments aside from the university life.

Finally, I want to thank my family, my siblings Julia and Felix Rettenwander as well as my parents Sabine and Johann Rettenwander for their enduring support and encouragement during my studies. Without their help this master´s thesis would not have been possible.

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ABSTRACT

Dynamic polymers are a special class of functional polymers containing reversible interactions within the polymer chain that can restore the material properties upon taking damage and adapt to a change of conditions in the chemical or physical surroundings. These dynamic properties are currently gaining more and more interest in research as there is no replacement of damaged materials necessary since it has self-healing properties. The reversible character of the inter-chain interactions allows the bonds to reform after being broken. However, for current approaches the strength of the reversible interaction is solely determined by the system used without any adjustment of the functional groups possible. Therefore, in this master´s thesis a two-component system was developed, which is based on Lewis acid-base interactions with adaptable properties. The polymeric backbone for both components was chosen to be polydimethylsiloxane which provides outstanding properties like thermal resistance and the possibility to modify their functional groups. As the Lewis acid component, triphenyl borane groups are used as its acidity can be varied by adding electron-shifting substituents to the phenylic rings, which allows adaptation of the properties of the resulting material. The functional group was prepared using a Grignard reaction and afterwards connected to the silicone via a hydrosilylation reaction. Hydrosilylation was also utilized to incorporate pyridine in siloxane to obtain the Lewis base counterpart. Mixing of the two components resulted in an immediate network formation observable as an increase in viscosity.

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KURZFASSUNG

Dynamische Polymere stellen eine besondere Klasse funktioneller Polymere dar, welche reversible Wechselwirkungen zwischen den Komponenten ihrer Polymerketten aufbauen. Durch diese kann sich beschädigtes Material selbstständig regenerieren. Zudem kann sich das Polymer an die Umgebungsbedingungen anpassen. Wegen dieser besonderen Eigenschaften ist kein Austausch des beschädigten Materials nötig, weshalb an Polymeren dieser Art aktuell viel Forschung betrieben wird. Bei den derzeit etablierten Systemen ist es jedoch nicht möglich, die Eigenschaften des Materials einzustellen, weil die Stärke der reversiblen Bindungen lediglich von der Art des verwendeten Systems abhängig ist. Das Ziel dieser Masterarbeit ist daher die Entwicklung eines zwei-Komponenten Systems, das auf Lewis-Säure-Base Wechselwirkungen beruht und dessen Eigenschaften angepasst werden können. Diese Gruppen werden in Polydimethylsiloxanpolymere integriert, weil diese viele Vorteile wie beispielsweise Temperaturbeständigkeit mit sich bringen und einfach funktionalisiert werden können. Als Lewis-Säure Komponente wird Triphenylboran verwendet, weil durch Substituenten an den Phenylringen eine Einstellung der Säurestärke möglich ist. Diese Komponente wurde durch eine Grignard Reaktion synthetisiert und durch eine Hydrosilylierungsreaktion mit der Siloxankette verknüpft. Mit dieser Reaktion konnten zudem Siloxane mit Pyridingruppen funktionalisiert werden, wodurch die basische Komponente erhalten wurde. Durch das Mischen von Lewis-Säure und Lewis-Base Komponenten entsteht augenblicklich ein Polymernetzwerk, das sich durch eine deutlich erhöhte Viskosität im Vergleich zu den einzelnen Komponenten auszeichnet.

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4. Conclusion ... 66 5. References ... 67 6. Appendix ... 71 6.1. Diphenylborinic acid ... 71 6.2. Diphenylborosiloxane ... 72 6.3. 4-bromophenethyl terminated PDMS. ... 73 6.4. 4-(diphenylborane)phenethyl terminated PDMS ... 75 6.5. 4-Styryl-diphenylborane ammoniate ... 77 6.6. 4-Styryl-diphenyl borane ... 78 6.7. 4-(diphenylborane-ammoniate)phenethyl terminated PDMS ... 79

6.8. Poly(ethenylpyridylmethyl) siloxane dimethylsiloxane copolymer ... 81

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LIST OF FIGURES

Figure 1. Diels-Alder reaction of two functional groups, where R1, R2=polymeric

chains ... 14

Figure 2. Reaction equation of the equilibrium of anthracene and its dimer, where R=polymeric chain ... 15

Figure 3. Proposed mechanism of disulphide exchange ... 16

Figure 4. Illustration of the different imine reactions: (a) imine condensation/hydrolysis, (b) imine exchange, and (c) imine metathesis. ... 16

Figure 5. Schematic drawing of the ionic interaction self-healing polymer system ... 17

Figure 6. Schematic drawing of host-guest interaction polymers ... 17

Figure 7. Boron-containing polymer described by Frieder et al. ... 18

Figure 8. Siloxane-based Lewis acid-base pair interaction ... 19

Figure 9. Polymer containing boron within its main-chain ... 20

Figure 10. Proposed reaction pathway for Grignard reactions... 21

Figure 11. Solvation of a Grignard reagent in diethyl ether. ... 22

Figure 12. Synthetical route to vinyl-boranes proposed by Wang et al.. ... 23

Figure 13. General synthesis of polysiloxanes following the Rochow Process ... 24

Figure 14. Chalk-Harrod mechanism of the hydrosilylation reaction ... 25

Figure 15. Conversion of the dehydration reaction to diphenylborosiloxane 4 visualized by 11B NMR spectroscopy (96 MHz, CDCl3). ... 46

Figure 16. FT-IR spectrum of DMS-H11 6 and 4-bromophenethyl terminated PDMS 7 showing the decreasing Si-H band at 2200 cm-1 during the hydrosilylation reaction. ... 48

Figure 17. 1H NMR shift of aromatic protons a) before and b) after the Grignard reaction of compound 8. ... 49

Figure 18. FT-IR spectrum of DMS-H11 6 and 4-(diphenylborane-ammoniate)phenethyl terminated PDMS 14 showing the decreasin Si-H band at 2200 cm-1 during the hydrosilylation. ... 53

Figure 19. 29Si NMR spectrum of a) DMS-H11 6 and b) 4-(diphenylborane-ammoniate)phenethyl terminated PDMS 14. ... 54

Figure 20. 11B NMR spectrum of a) 4-(diphenylborane-ammoniate)phenethyl terminated PDMS 14 and b) 4-(diphenylborane)phenethyl terminated PDMS 9. ... 56

Figure 21. Comparison of the different reaction conditions visualised by the regressing Si-H peak at 2200 cm-1. ... 58

Figure 22. Comparison of the Si-H group conversion of 17 and 19. ... 61

Figure 23. 11B NMR spectroscopy of compound 9 a) before and b) after the addition of pyridine. ... 63

Figure 24. Resulting network of compound 9 in combination with any Lewis base (LB) containing silicone. ... 64

Figure 25. Pictures of the polymeric networks obtained from mixing LA and LB components. ... 65

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Figure 26. Diphenylborinic acid 2. ... 71

Figure 27. 1H NMR spectrum of 2. ... 71

Figure 28. 11B NMR spectrum of 2. ... 71

Figure 29. Diphenylborosiloxane 4. ... 72

Figure 32. 4-bromophenethyl terminated PDMS 7. ... 73

Figure 34. 29Si spectrum of 7. ... 73

Figure 35. ATR FT-IR spectrum of 7. ... 74

Figure 36. 4-(diphenylborane)phenethyl terminated PDMS 9. ... 75

Figure 39. 29Si NMR spectrum of 9. ... 76

Figure 41. 4-styryl-diphenyl borane ammoniate 12. ... 77

Figure 44. 4-Styryl-diphenyl borane 13. ... 78

Figure 47. 4-(diphenylborane-ammoniate)phenethyl terminated PDMS 14. ... 79

Figure 51. Poly(ethenylpyridylmethyl) siloxane dimethylsiloxane copolymer 21. ... 81

Figure 52. 1H NMR spectrum of 21 ... 81

Figure 54. Poly(isonicotinamidylmethyl)siloxane dimethylsiloxane copolymer 24. ... 83

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LIST OF SCHEMES

Scheme 1. Synthesis of diphenylborinic acid 2. ... 29 Scheme 2. Dehydration reaction to obtain diphenyl borodimethylsiloxane 4. ... 30 Scheme 3. Hydrosilylation reaction to obtain 4-bromophenethyl terminated PDMS 7.

... 31

Scheme 4. Grignard reaction of 7 to the organometallic 4-(diphenylborane)phenethyl

terminated PDMS 8. ... 33

Scheme 5. Grignard reaction of 4-chlorostyrene 9 with 2-aminoethyl diphenylborate 1. ... 34 Scheme 6. Barbier reaction to obtain 4-styryl-diphenylborane ammoniate 11. ... 35 Scheme 7. Ammonium chloride cleavage to obtain 4-styryl diphenylborane 12. ... 37 Scheme 8. Hydrosilylation reaction to 4-(diphenylborane)phenethyl terminated

PDMS 13. ... 38

Scheme 9. Hydrosilylation reaction to 4-(diphenylborane ammoniate)phenethyl

Scheme 10. Ammonium chloride cleavage to obtain 4-(diphenylborane)phenethyl

Scheme 11. Hydrosilylation reaction to obtain poly(pyridylmethyl)siloxane 17. ... 41 Scheme 12. Hydrosilylation reaction to obtain poly(ethenylimidazolylmethyl)siloxane 19. ... 42 Scheme 13. Hydrosilylation reaction to pyridylmethylsiloxane 21. ... 43 Scheme 14. Substitution reaction of AMS-162 22 with isonicotinoylchloride

hydrochloride 23 to obtain poly(isonicotinamidylmethylsiloxane) dimethylsiloxane copolymer 24. ... 44

Scheme 15. Synthesis of diphenylborinic acid 2 via the acid hydrolysis of

2-aminoethyl diphenyl borinate 1. ... 45

Scheme 16. Dehydration reaction of diphenylborinic acid 2 and hydroxyl terminated

PDMS 3 to obtain diphenylborosiloxane 4. ... 45

Scheme 17. Hydrosilylation reaction of 4-bromostyrene 5 and DMS-H11 6 to obtain

4-bromophenethyl terminated PDMS 7. ... 47

Scheme 18. Grignard reaction of 8 to obtain 4-(diphenylborane)phenethyl terminated

PDMS 9. ... 48

Scheme 19. Synthesis of 4-styryl-diphenylborane ammoniate 12 via Grignard

reaction of either 4-chlorostyrene 10 or 4-bromostyrene 5. ... 51

Scheme 20. Hydrosilylation reaction to 4-(diphenylborane-ammoniate)phenethyl

Scheme 21. Ammonium chloride cleavage of 14 to obtain 4-(diphenylborane)phenethyl terminated PDMS 9. ... 55

Scheme 22. Hydrosilylation reaction of PMHS 15 and 4-viylpyridine 16 to obtain

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December 2019 Patrick Rettenwander 11/83 Scheme 23. Hydrosilylation reaction of HMS-301 20 and 4-vinylpyridine 16 to obtain

poly(ethenylpyridylmethyl) siloxane dimethyl-siloxane copolymer 21. ... 59

Scheme 24. Hydrosilylation reaction of PMHS 15 and 1-vinylimidazole 18 to obtain

poly(ethenyl-1-imidazolylmethyl)siloxane 19. ... 60

Scheme 25. Substitution reaction of AMS-162 22 with isonicotinoylchloride

hydrochloride 23 to obtain poly(isonicotinamidylmethylsiloxane) dimethylsiloxane copolymer 24. ... 62

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LIST OF TABLES

Table 1. Used Abbreviations. ... 13 Table 2. Chemicals Used during the thesis. ... 27 Table 3. Yields of 12 of the Grignard reaction of reactant 10. ... 51 Table 4. Reaction conditions and corresponding conversion of Si-H groups for the

hydrosilylation reaction of PMHS with p-vinylpyridine. ... 58

Table 5. Reaction conditions and corresponding conversion of Si-H groups for the

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ABBREVIATIONS

Abbreviations used in this thesis are listed in table 1.

Table 1. Used Abbreviations.

Abbreviation Definition abs. Absolute AM Anti-Markovnikoff AMS Aminopropylmethylsiloxane anh. Anhydrous Ar Aryl

ATR Attenuated total reflection

br Broad

DA Diels-Alder

DCM Dichloromethane

eq. Equivalent

ESI Electrospray ionisation

Et2O Diethyl ether FT Fourier-transform HMS Hydromethylsiloxane IR Infrared M Markovnikoff MeOH Methanol MS Mass spectrometry

NMR Nuclear magnetic resonance

p.a. Per analysis

Ph Phenyl

PMHS Poly(methylhydrosiloxane)

RAFT Reversible addition-fragmentation chain transfer

RT Room temperature

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1. Introduction

1.1. Dynamic Polymers

Dynamic polymers, which are also denoted as dynamers [1-3], generally describe a polymeric system that is linked reversibly by the interaction of the components of the polymer chain [1, 2]. Therefore, it can adapt to external stimuli, like a change in temperature or its chemical surrounding [1-3]. This adaptation usually occurs as a change in the stress/strain behaviour or the shape of the material [3]. The dynamic character of the interaction within the material leads to a self-healing property of the material when the condition change is reverted [2]. Self-healing materials are of great importance in material science since there is no need to replace the damaged materials [4, 5]. To obtain these properties, it is required to insert functional groups in the polymer that enable reversible linkages [2].

The interactions can be of physical nature such as supramolecular chemistry with non-covalent interactions within the polymer or chemical, which includes covalent bondings [1-5]. The chemical interactions require the formation of a bond, which is responsive to external stimuli, as well as preserving the stability of the residual covalent bonds. Known approaches for this are imine bonds, disulphide bonds, reverse-radical reaction and Diels-Alder (DA) reactions. The latter one is exclusively dependent on the temperature [5, 6]. The classical [4+2] cycloaddition DA reaction of an electron-rich diene with an electron-deficient dienophile occurs at temperatures around 60 °C, whereas the equilibrium shifts to the reactants at 110 °C, as the retro DA becomes the predominant reaction [6, 7]. An example for a reversible crosslinking by using a DA reaction is shown in figure 1 [4].

DA reactions provide a number of advantages including the lack of side-products, yielding a 3D network from 2D reagents and providing a structural diversity.

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December 2019 Patrick Rettenwander 15/83 However, usually a heating above the glass transition temperature of the polymer is needed, which leads to a permanent deformation of the bulk material. This complicates the use in mechanically demanding applications [8]. Hence, there are also approaches using a two-component system of a substituted polymer and a hardener to achieve reversible DA connections at room temperature. To achieve this, bis(fulvene) terminated polyethylene glycol was combined with bis(dicyanofumarate). This leads to a quick breaking and reforming of the covalent bonds at 25 °C [4, 9].

A very similar approach is the [4+4] cycloaddition of anthracene derivatives as a photoresponsive method. In this system the dimerisation of anthracene during exposure to low-energy radiation is utilised. This can be reverted by irradiation with lower wavelengths, which allows a comparatively easy control of the reaction´s equilibrium, which is schematically drawn in figure 2 [10].

Figure 2. Reaction equation of the equilibrium of anthracene and its dimer, where R = polymeric

chain [10].

An alternative method for covalent crosslinking is the redox reaction between thiol and disulphide, which also establish a dynamic equilibrium, whereby the corresponding redox equation is displayed in figure 3 [11, 12]. The exchange is based on a nucleophilic attack of a thiolate anion on a disulphide bond resulting in a new disulphide bond and a different anion [11]. In general, the reaction can be accelerated by adding a catalyst to the system. However, even without any external factors, the reaction occurs at ambient conditions. The system can also respond to mechanical stress as the disulphide bonds break forming thiyl radicals. Those quickly exchange with other disulphide bonds. Consequently, a part of the initial energy is released which additionally prevents material damage. A further advantage of this system is the possibility to generate branched functionalized polymers which are

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Figure 3. Proposed mechanism of disulphide exchange [11].

more accessible to bond formation and allow an easier regeneration of the system [12].

Other examples for covalent dynamic bonds are imine bonds. Those are obtained by the condensation reaction of an aldehyde with an amine group [13]. Usually, di- or tri-functional aldehydes are combined with di-functional primary amines to create a 3-D crosslinked network. The functional groups are both mostly connected to aromatic systems [14] providing a system with several advantages such as mild equilibrium conditions and a catalyst-free reaction [15]. Especially the selectivity of the reaction and its resistance to solvents and poisoning represents an important part [13, 15]. From a chemical point of view the system consists of three different equilibrium reactions rather than just one for other dynamic polymers. Those are hydrolysis/condensation, which was already mentioned previously, imine exchange and imine metathesis [13, 15, 16] as it is displayed in figure 4 [16].

Figure 4. Illustration of the different imine reactions: (a) imine condensation/hydrolysis, (b) imine

exchange, and (c) imine metathesis [16].

In contrast, physical interactions are non-covalent connections between the functional groups. The most important examples are hydrogen bonding, host-guest interactions, π-π stacking, and metal-ion interactions [5, 17]. Also ion-ion interactions are described in the literature [18]. In this system strong covalent bonds are combined with comparatively weak ionic interactions of anionic and cationic groups, which are inserted by random copolymerisation into the polymer chain. The resulting system is schematically drawn in figure 5. It provides the advantage of adaptation to external stimuli, as it was described before, while retaining some of its mechanical properties due to the stronger covalent bonds, which are not affected by the condition changes [18].

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Figure 5. Schematic drawing of the ionic interaction self-healing polymer system [17].

The principle of a host-guest interaction is the inclusion of a small molecule (guest) in a cyclic structure (host) via hydrophobic or hydrophilic interactions to form an encapsulation complex [19]. This complex is schematically drawn in figure 6 [20]. The most important host functionalities are cyclodextrins [20, 21], cucurbiturils [20-22], calixarenes, and pillarenes [20]. Cyclodextrins are by far the most common ones [21, 22] since they are produced on an industrial scale and provide advantages like biocompatibility and degradability. Additionally, they are already well known because of their utilization in food industry [20]. A typical guest modification to interact with cyclodextrins are adamantane functionalized polymers [20, 21]. Usually, host and guest substituted chains are produced separately to avoid intra-chain interactions, which would prevent gel formation. The strength of the gel can be directly referred to the association constant Kass of the cyclodextrin host-guest pair [21].

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1.2. Boron-containing polymers

A promising approach to obtain dynamic polymers is the insertion of boron compounds in the polymer chain, as described by Jäkle et al. [23]. In contrast to other group 13 elements, boron containing organic polymers are well known and used in variable applications e.g. flame-retardants, components of lithium-ion batteries, supramolecular materials, sensors for anions [23, 24] and optoelectronic materials [23-25]. Therefore, boron compounds can be incorporated in polymers like polystyrene while retaining the Lewis-acidity [25, 26]. The incorporation of boron was done by post-polymerisation modification in this approach. To achieve this, a silicon-boron exchange is performed to obtain the boron functionalized polymer. The resulting polymer is displayed in figure 7 [26]. Simultaneously, also the reversible addition-fragmentation chain-transfer (RAFT) polymerisation of boronic ester-containing monomers was recently described [27]. The electron deficiency of those polymers results in a high delocalization of the electrons within the polymer chains, which leads to colourisation [25].

The major aspect of such systems is their ability to undergo Lewis acid-base-pair complex formation, when a Lewis base containing compound is added. Using this donor-acceptor interactions, supramolecular structures with the same properties as described above can be synthesized [28, 29]. One advantage of this system is that the healing progress occurs much quicker than it is the case for other self-healing gels [28]. However, the major advantage is the flexibility in the use of the boron Lewis acid. Varying the substituents the acidity of the boron compounds can be adapted. Therefore, the strength of the reversible intermolecular bonds can also be adapted as the Lewis acid-base bonding energy is changed [29].

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Figure 8. Siloxane-based Lewis acid-base pair interaction [30].

Dodge et al. describes a system of Lewis acid-base pairs in polymeric polydimethylsiloxane (PDMS) as a self-healing polymeric rubber material consisting of two components. Commercially available vinyl boron compounds are added to a hydride end-capped PDMS polymer chain via a hydrosilylation reaction. As a Lewis base counter-part a PDMS copolymer containing amine side-groups is used. The resulting system is shown in figure 8. Depending on the acid/base ratio of the components elastomeric rubbers of different hardness and physical properties are obtained [30]. The dynamic B-N bond is able to regenerate after being broken without any external influences [31]. However, the disadvantage of this system is that there is no adaptation of the strength of the donor acceptor possible as it is limited to commercially available reagents. The only variation can be achieved by adapting the ratio and amount of functional groups in the elastomer [30]. The same concept is used by Cao et al., but with both types of functional groups within the same polymer chain rather than a two-component system [31]

A similar system is the base for the commercial product D3O, which is a highly shock-absorbing material. To obtain the product, polyborodimethylsiloxane (PBDMS) is suspended in a polyurethane matrix. The result is a dispersed elastomer with high compressibility. Those are used in the military or in sports´ protection wear [32].

Some of the classical approaches are polymers containing boron within their main-chain. Those are obtained by hydroboration addition to a double or triple bond to achieve polymerisation. They contain a comparatively large number of boron atoms, which, however, cannot be modified and are less accessible than side-chain functionalities. A typical system from the literature is shown in figure 9 [24].

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Figure 9. Polymer containing boron within its main-chain [24].

1.3. The Grignard reaction

The Grignard reaction is named after its discoverer Victor Grignard, who was awarded a Nobel Prize in 1912 for the development of organo-magnesium compounds and its synthetic application [33, 34]. It refers to the reaction of an organic halide, most commonly alkyl or aryl halides, with elemental magnesium in ether suspension to the Grignard reagent, where the magnesium is inserted in the carbon-halogen bond to obtain the organomagnesium compound [33, 35, 36] which leads to a change in oxidation state from Mg(0) to Mg(II) [36]. In most cases a slight excess of magnesium is used [34]. The reaction equation is shown in equation 1 [33, 35]. As a result, the electrophilic α-carbon is converted into a nucleophile, the reactivity of the substrate is inverted. Therefore, the reaction is a type of polar inversion [35, 37].

where X = Cl, Br, I

R = Alkane, Alkene, Alkyne, Arene

The reaction occurs at the metallic surface of the magnesium. An electron from the metal is transferred to the halide resulting in a radical alkyl or aryl species [36, 37]. The radical can either stay absorbed at the metal surface or diffuse into the solution. Additionally, at the surface the reunion of the radical species occurs, resulting in the Grignard reagent. The reaction pathway is illustrated in figure 10 [37].

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Figure 10. Proposed reaction pathway for Grignard reactions [37].

In the solution there is a substrate-depending equilibrium between the monoalkylated and the dialkylated magnesium complexes called the Schlenk equilibrium, which is illustrated in equation 2 [34, 35, 38]. The equilibrium is strongly influenced by the solvent used. In tetrahydrofuran (THF) the equilibrium is predominantly in between both forms (II) whereas diethyl ether (Et2O) favours the left

side (I). The right side (III) is preferred in basic solvents like triethylamine [38].

The progress of the reaction can be observed by the decreasing amount of metallic magnesium in the suspension. Since the conversion to the Grignard reagent occurs at the surface, the formation of an oxide layer at the metal can prevent the reaction from initiating [37]. Due to this layer there is usually an induction period at the beginning of the reaction [34, 36]. To counteract this problem, it might sometimes be necessary to add oxidizing reagents, e.g. iodine, to the reaction mixture to partly remove the oxide layer and increase the reactivity of the magnesium [37]. Also, residual moisture in the reaction vessel can inhibit the reaction´s initiation [34]. The starting of the reaction can be observed by the generation of heat and therefore an increasing temperature of the reaction mixture. This results from the exothermal Grignard reaction. Consequently, it is necessary to slowly add the halide to the magnesium suspension [34].

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Figure 11. Solvation of a Grignard reagent in diethyl ether [34].

In terms of halides, chloride, bromide, and iodide compounds can be used for this reaction with the latter two being far more reactive than the chloride, which only reacts very slowly on the magnesium surface [34], but results in the highest yields [38]. The most commonly used halogen is bromine [34], whereas concerning solvents, diethyl ether and tetrahydrofuran are the most common ones. THF is usually used for less reactive reagents, because the higher boiling point allows higher temperatures to ensure the conversion of the substrates. The use of ether as a solvent is necessary because of the solvation of the Grignard reagent as ether complexes, since Mg(II) prefers tetrahedral structures [34, 36] like the one shown in figure 11. The magnesium acts as a Lewis acid to form a complex with one of the lone pairs of the ether´s oxygen atom, which acts as Lewis base [34].

Grignard reagents are frequently used as a nucleophile to add an organic substrate to polar double bonds forming a carbon-carbon bond. The most commonly applied reaction is the reduction of a carbonyl group to an alcoholate. Besides that, also reactions with carbon dioxide, nitriles and other polar functional groups like C=S are possible [37].

Moreover, reactions with protic deuterated reagents allows the controlled insertion of deuterium into organic compounds, which is shown in equation 3 [37].

Also, the reaction with organohalides to form a carbon-carbon single bond is frequently applied. The reaction equation is depicted in equation 4 [35].

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Figure 12. Synthetic route to vinyl-boranes proposed by Wang et al. [28].

However, in this thesis, the Grignard reagent will be used for a nucleophilic substitution reaction at a boron atom as it is proposed by Wang et al [28]. As reagent, a vinylic compound is necessary for further steps as shown in chapter 1.5. The formation of the reagent as well as the following substitution reaction are shown in figure 12 [28].

It needs to be considered that allylic Grignard reagents undergo side-reactions like dimerisations [36]. Also, for para-bromostyrene it is known that there is a self-polymerisation proceeding under anionic conditions as they are present during the Grignard reaction [39]. It is also well documented that Grignard reagents can act as an initiator for anionic polymerisations of vinylic compounds [40-42]. However, the Grignard compound is only a weak initiator for polymerisation reactions as magnesium is not as electropositive as lithium. Consequently, it should not lead to a polymerisation, as long as there are no electron-withdrawing groups present at the vinyl group, which is not the case for the reaction mentioned above [42].

1.4. Silicones

Silicone is the general term used for polymeric materials with a silicon-oxygen backbone and organic groups directly attached to the silicon atom. The most common silicone is polydimethylsiloxane, which exclusively contains methyl groups as organic substituents [43, 44]. Therefore, silicones are polymeric hybrids of inorganic and organic components and intermediates between silicates and organic polymers [44]. The term silicone originates from the formerly believed analogy to ketones as the R2SiO building blocks formally correspond to the ketone structure

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December 2019 Patrick Rettenwander 24/83 R2CO. However, silicon-oxygen double bonds are unstable so there is no isolated

monomeric R2SiO known [44, 45].

Figure 13. General synthesis of polysiloxanes following the Rochow Process [48].

For scientific nomenclatures the term siloxane is used nowadays rather than silicone for compounds containing a Si-O-Si backbone while silicone is used to denote technical products based on siloxanes [44]. This kind of nomenclature will also be the one used in this thesis. Silicones provide a number of advantages in comparison to common, purely organic polymers. Due to the very stable Si-O bonds silicones are thermally far more resistant. With glass transition temperatures of down to -120 °C and flash points of above 300 °C silicones retain their properties over a large temperature range. They are also liquid even at large molecular weights [45, 46]. Additionally, silicones are highly permeable to gases [46, 47] and very resistant to radiation and oxidation in comparison to organic polymers [31, 45]. However, they are more sensitive to degradation by strong acids or bases [45].

Silicones are produced out of diorganic dichlorosilanes. Those are manufactured via the Rochow Process using elemental silicon und organochlorides as staring materials. Hydrolysis and subsequent condensation give the siloxane´s characeristic reapeating unit. The reaction is displayed in figure 13. This route provides the advantage that the dichlorosilane precursor can be purified by distillation. The addition of differently substituted dichlorosilanes allows the synthesis of random copolymers with varying functional groups within the polymer chains [48].

1.5. The hydrosilylation reaction

Hydrosilylation is the addition of organic multiple bonds to a silicon hydride bond. In this reaction it does not matter whether the silicon is polymer-bound or present as an inorganic compound [49]. Nevertheless, the most convenient application can be found in silicone modifications. It is used for curing of silicone rubbers to form a three dimensional network between silicone chains by the reaction of hydride- and vinyl-functionalized silicones. Additionally, the reaction is often applied for post-polymerisation functionalization [50]. It is catalysed by a variety of

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December 2019 Patrick Rettenwander 25/83 transition metals with Speier´s catalyst H2[PtCl6] [50, 51] and Karstedt´s catalyst

[Pt2(sym-tetramethyldivinyldisiloxane)3] [51] being the most frequently used ones.

The mechanism of the reaction was proposed by Chalk and Harrod considering Pt(0) as the active catalyst. It consists of four major steps that are shown in figure 14, being (1) oxidative addition of the metal by the Si-H group, (2) alkene coordination, (3) alkene insertion into the Si-H bond, and (4) the reductive elimination of the addition product [51].

Figure 14. Chalk-Harrod mechanism of the hydrosilylation reaction [51].

In this thesis, only the hydrosilylation of vinylic aromatic compounds to hydride containing, both end-capped and internal, silicones will be investigated, as it is already documented in many sources [52-55].

1.6. Aim of the thesis

The aim of this thesis is to synthesize a silicone elastomer with self-healing properties. Therefore, a two-component system that is based on PDMS should be introduced. The main component should be a PDMS chain which contains pyridine functionalities that act as Lewis bases. Those are connected by shorter PDMS chains that are end-capped by boron-functional groups that act as Lewis acids. As a result, the crosslinking occurs by the formation of a LA-LB complex. The final product should be an elastomer that responds very quickly to external pressure changes as the crosslinking breaks and reforms.

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2. Experimental

2.1. General

Unless otherwise stated, all reactions were performed under inert conditions using nitrogen and Schlenk techniques. Chemicals and solvents used were purchased from commercial suppliers and used as received if not specified any differently. 4-Chlorostyrene, 4-bromostyrene, DMS-H11, PMHS, HMS-301, and 1,2-dibromoethane were dried with molecular sieves (3 Å) prior to usage. 1-Vinylimidazole was dried prior to use by magnesium sulphate. Magnesium turnings were stored and weighed within a glovebox. 1H, 11B, and 29Si nuclear magnetic resonance (NMR) spectroscopy measurements were performed on a Bruker Advance III 300 MHz spectrometer. 1H NMR spectra were calibrated using the solvent´s residual peak. Electrospray ionisation mass spectrometry (ESI-MS) was performed on a Shimadzu 20 A using 97 % methanol, 10 mM ammonium formiate, and 3 % water as eluent. The measurement was done in a range of m/z = 100-700. Fourier-transform infrared spectra were measured with an attenuated total reflection device on a Perkin Elmer 100 series. Per measurement 16 scans were recorded.

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2.2. Used Chemicals

Chemicals and solvents used during this thesis are listed along with their suppliers and purity in table 2.

Table 2. Chemicals used during the thesis.

Substrate Supplier Purity / %

Acetone VWR 100.0

Acetone-d6 Eurisotop 99.8

Ammonium chloride VWR 100.0

2-Aminoethyl diphenylborinate TCI 98

2-Aminoethyl diphenylborinate Fluorochem 98

AMS-162 Fluorochem -

4-Bromostyrene TCI 95

Chloroform D Eurisotop 99.8

4-Chlorostyrene Fluorochem 97

1,2-Dibromoethane Sigma Aldrich 99

Dibromomethane TCI 99

Dichloromethane Chem-Lab 99.8

Dichloromethane anh. Alfa Aesar 99.7

Diethyl ether VWR 99.8

Diethyl ether anh. - -

DMS-H11 Fluorochem -

Dimethylsulphoxide-d6 Eurisotop 99.8

Ethanol abs. Chem-Lab 100.0

Ethyl acetate VWR 99.9

n-Hexane VWR 98

n-Heptane VWR 99.9

HMS-301 Fluorochem -

Hyrochloric acid VWR 37

Hydrochloric acid anh. in Et2O TCI -

Iodine Alfa Aesar 99.0

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Continuation Table 2. Chemicals used during the thesis.

Substrate Supplier Purity / %

Magnesium sulphate VWR 99.9

Magnesium turnings Fluorochem 99

Methanol VWR 99.9

Molecular sieves 3Å Alfa Aesar -

Platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution

in Xylenes (Karstedt´s catalyst)

Sigma Aldrich 2

PDMS hydroxy terminated abcr -

Poly(methylhydrosiloxane) Sigma Aldrich -

Potassium chloride J.T. Baker 99.0

Pyridine J.T. Baker 99.0

Sodium hydroxide J.T. Baker 98

Tetrahydrofurane VWR 99.9

Tetrahydrofurane anh. Alfa Aesar 99.8

Toluene VWR 100.0

Toluene anh. Sigma Aldrich 99.8

Triethyl amine VWR 99.0

1-Vinylimidazole Alfa Aesar 99

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2.3. Synthesis of diphenylborinic acid

Scheme 1. Synthesis of diphenylborinic acid 2.

At ambient conditions compound 2 was synthesized according to the literature [56]. 2-Aminoethyldiphenyl borinate 1 (500 mg, 2.22 mmol) was dissolved in acetone/methanol (1:1, 10 mL). Aqueous HCl (1M, 10 mL) was added. After a few seconds a slight heat generation was observed. The solution was stirred at RT for 2 h. Afterwards it was extracted with EtOAc (3 × 15 mL). The combined organic phases were dried with magnesium sulphate. The solvent was removed on a rotary evaporator to obtain the product 2 as a yellow oil.

Yield: 301 mg (1.65 mmol, 76 %) 1_{H NMR (300 MHz, DMSO-d} 6) δ / ppm: 9.98 (s, 1H, OH), 7.70 (dd, J=6.4 Hz, 1.4 Hz, 4H, ArH), 7.52-7.36 (m, 6H, ArH) 11_{B NMR (96 MHz, DMSO-d} 6) δ / ppm: 41.5 (br s)

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2.4. Synthesis of diphenyl borodimethylsiloxane

Scheme 2. Dehydration reaction to obtain diphenyl borodimethylsiloxane 4.

Under ambient conditions diphenylborinic acid 2 (180 mg, 0.989 mmol) was mixed with hydroxyl-terminated PDMS 3 (118 mg, 0.489 mmol). The mixture was dissolved in xylene (1 mL) and heated up to 70 °C under reduced pressure for 4 h. The product 4 was obtained as a yellow oil.

Yield: 276 mg (0.486 mmol, 99 %) 1_{H NMR (300 MHz, CDCl} 3) δ / ppm: 8.26 (dd, J=1.4 Hz, 6.4 Hz, 4H, ArH), 7.57-7.52 (m, 4H, ArH), 0.09 (s, 18H, SiCH3) 11_{B NMR (96 MHz, CDCl} 3) δ / ppm: 29.7 (s)

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2.5. Synthesis of 4-bromophenethyl terminated PDMS

Scheme 3. Hydrosilylation reaction to obtain 4-bromophenethyl terminated PDMS 7.

The procedure to obtain 7 was adapted from the literature [57]. DMS-H11 6 (2.001 g, 2.001 mmol) was transferred into a Schlenk flask and degassed by vacuum for 5 min. The siloxane was dissolved in anhydrous toluene (10 mL). A solution of platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane in xylenes (Karstedt´s catalyst, 2 %, ~50 µL) in anh. toluene (2 mL) was added which caused the reaction mixture to turn bright yellow after a few minutes. 4-Bromostyrene (0.84 g, 0.60 mL, 4.6 mmol) was added dropwise over 10 minutes. The reaction mixture was stirred at RT for 3 h until IR spectroscopy indicated complete conversion of Si-H groups. Afterwards, the solvent was removed on a rotary evaporator. The residual brown oil was dissolved in

n-heptane (30 mL) and washed with deionized water (3 × 20 mL). The organic phase

was dried with magnesium sulphate and evaporated to dryness on a rotary evaporator. The product, 4-bromophenethyl terminated PDMS 7, was obtained as a clear, pale yellow oil. The product was a mixture of Markovnikoff (M) and anti-Markovnikoff (AM) addition products in a ratio of about 1:2.

Yield: 2.179 g (1.596 mmol, 80 %)

FT-IR ν / cm-1: 2962 (C-Haliphatic), 1596, 1488, 1401 (C=Caromatic), 1258 (Si-C),

1021 (Si-O) 1_{H NMR AM (300 MHz, CDCl} 3) δ / ppm: 7.48-7.27 (m, 2H, ArH), 7.07 (d, J=8.4 Hz, 2H, ArH), 2.66-2.56 (m, 2H, CH2), 0.95-0.81 (m, 2H, CH2), 0.20-(-0.13) (m, 45H, SiCH3) 1_{H NMR M (300 MHz, CDCl} 3) δ / ppm: 7.48-7.27 (m, 2H, ArH), 6.96 (d, J=8.4 Hz, 2H, ArH), 2.14 (q, J=7.6 Hz, 7.5 Hz, 1H, CH), 1.34 (d, J=7.5 Hz, 3H, CH3)

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29_{Si NMR AM (60 MHz, CDCl}

3) δ / ppm: 6.9, -21.1, -21.9, -22.0 29_{Si NMR M (60 MHz, CDCl}

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2.6. Synthesis of 4-(diphenylborane)phenethyl terminated PDMS

Scheme 4. Grignard reaction of 7 to 4-(diphenylborane)phenethyl terminated PDMS 9.

Magnesium turnings (110 mg, 4.53 mmol) were mixed with anh. THF (20 mL). 1,2-Dibromoethane (~50 µL) was added and the reaction mixture was stirred at RT for 30 min until a grey colour was observed. Compound 7 (1.00 g, 0.732 mmol) was added dropwise over 5 min and the solution was stirred at RT for 1 h. Afterwards, the solution was refluxed for 2 h. The solution was allowed to return to room temperature and then cooled in an ice bath. A solution of 2-aminoethyl diphenylborinate 1 (330 mg, 1.47 mmol) in anh. THF (10 mL) was added over 10 min. The solution was stirred at 0 °C for 1 h, then at RT for 16 h. Residual magnesium was removed by cannula filtration. The reaction was then quenched with anhydrous HCl in Et2O (1M,

4 mL). The resulting white precipitate was removed by cannula filtration. Solvent and excess HCl were removed by vacuum to give the product 9 as a yellow oil.

Yield: 1.10 g (0.651 mmol, 87 %) 1_{H NMR (300 MHz, CDCl} 3) δ / ppm: 7.82 (dd, J=1.5 Hz, 6.4 Hz, 4H, ArH), 7.57-7.28 (m, 8H, ArH), 7.07 (d, J= 8.4 Hz, 2H, ArH), 2.66-2.57 (m, 2H, CH2), 0.93-0.84 (m, 2H, CH2), 0.15-0.00 (m, 72H, SiCH3) 11_{B NMR (96 MHz, CDCl} 3) δ / ppm: 45.6 (s)

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2.7. Synthesis of 4-styryl-diphenylborane ammoniate

2.7.1. Pathway via chlorostyrene

Scheme 5. Grignard reaction of 4-chlorostyrene 9 with 2-aminoethyl diphenylborate 1.

Compound 12 was synthesized following the literature [28]. Magnesium turnings (481 mg, 19.8 mmol) were mixed with 25 mL of anhydrous THF. 1,2-dibromoethane (0.14 mL, 0.30 g, 1.6 mmol) was added dropwise over 15 minutes which caused the solvent to turn grey. The mixture was refluxed for 40 min and then allowed to return to room temperature. 4-chlorostyrene 10 (1.8 mL, 2.1 g, 15 mmol) was added dropwise over a period of 30 min. The suspension was refluxed for additional 30 min and then stirred at RT for 1 h. The green-greyish solution was transferred dropwise over 20 min into a solution of 2-aminoethyl diphenylborate 1 (1.595 g, 7.089 mmol) in anhydrous THF (17 mL) at -78 °C. Afterwards, the solution was stirred at 0 °C for 1 h and finally stirred at RT for 2 h. The reaction was quenched by addition of saturated aqueous NH4Cl (25 mL) and the two phases

thoroughly stirred for 30 min. The aqueous phase was then extracted by diethyl ether (3 × 10 mL). The organic phases were combined, dried with magnesium sulphate, and the volatile solvents removed on a rotary evaporator. The residual yellow oil was transferred into n-heptane (20 mL). The resulting precipitate was collected by suction filtration as a yellowish solid.

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December 2019 Patrick Rettenwander 35/83 1_{H NMR (300 MHz, DMSO-d} 6) δ / ppm: 7.26-6.94 (m, 14H, ArH), 6.65 (dd, J=10.9 Hz, 6.8 Hz, 1H, ArCHCH2), 5.68 (d, J=18.1 Hz, 1H, ArCHCH2), 5.60 (s, 3H, NH3), 5.09 (d, J=11.0 Hz, 1H, ArCHCH2) 11_{B NMR (96 MHz, DMSO-d} 6) δ / ppm: -5.3 (br s)

2.7.2. Pathway via bromostyrene

Scheme 6. Grignard reaction to obtain 4-styryl-diphenylborane ammoniate 12.

The procedure was derived from the literature [28]. 2-Aminoethyl diphenylborate 1 (2.005 g, 8.911 mmol) and magnesium turnings (569 mg, 23.4 mmol) were mixed with anhydrous THF (55 mL). 1,2-dibromoethane (0.05 mL) was added and the mixture stirred at RT for 30 min. During exclusion of light 4-bromostyrene 5 (0.28 g, 0.20 mL, 1.5 mmol) was added dropwise and the mixture stirred at RT for 30 min until a green colour of the solution was observed. The mixture was cooled in an ice bath and additional 4-bromostyrene 5 (2.52 g, 1.80 mL, 13.8 mmol) was added dropwise during 20 min. The mixture was stirred at this temperature for 2 h and afterwards at RT for additional 2 h. Saturated aqueous NH4Cl solution (30 mL) was added which caused a white colourisation of the solution.

The mixture was then stirred for 30 min. The aqueous phase was separated and extracted with diethyl ether (3 × 20 mL). The organic phases were combined, dried with magnesium sulphate, and the solvents removed on a rotary evaporator. The residual yellow oil was transferred into n-heptane (30 mL) to precipitate the crude product 12 as a white powder, which was collected by suction filtration (1.768 g,

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December 2019 Patrick Rettenwander 36/83 6.204 mmol, 71 %). The crude was recrystallized in toluene to obtain the purified product 12.

Yield: 1.114 g (3.909 mmol, 63 %, overall yield: 44 %)

1_{H NMR (300 MHz, DMSO-d} 6) δ / ppm: 7.25-6.97 (m, 14H, ArH), 6.65 (dd, J=10.9 Hz, 6.8 Hz, 1H, ArCHCH2), 5.68 (d, J=17.8 Hz, 1H, ArCHCH2), 5.60 (s, 3H, NH3), 5.09 (d, J=11.0 Hz, 1H, ArCHCH2) 11_{B NMR (96 MHz, DMSO-d} 6) δ / ppm: -5.0 (br s)

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2.8. Synthesis of 4-styryl diphenylborane

Scheme 7. Ammonium chloride cleavage to obtain 4-styryl diphenylborane 13.

The synthesis of 13 was performed in accordance to the literature [28]. 4-Styryl-diphenylborane ammoniate 12 (254 mg, 0.891 mmol) was mixed with anh. diethyl ether (5 mL). HCl in Et2O (1M, 5 mL) was added and the mixture stirred at RT

for 30 min. The resulting white precipitate was removed by cannula filtration. Solvent and excess HCl were removed under vacuum to yield the product 13 as a yellow oil.

Yield: 198 mg (7.39 mmol, 82 %) 1_{H NMR (300 MHz, CDCl} 3) δ / ppm: 7.96-7.73 (m, 6H, ArH), 7.60-7.41 (m, 8H, ArH), 6.80 (dd, J=10.8 Hz, 6.7 Hz, 1H, ArCHCH2), 5.88 (d, J=17.5 Hz, 1H, ArCHCH2), 5.36 (d, J=10.9 Hz, 1H, ArCHCH2) 11_{B NMR (96 MHz, CDCl} 3) δ / ppm: 45.5 (s)

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Scheme 8. Hydrosilylation reaction to 4-(diphenylborane)-phenethyl terminated PDMS 9.

The reaction was performed in analogy to the literature [30]. 4-Styryl diphenyl borane 13 (167 mg, 0.623 mmol) was dissolved in anhydrous toluene (8 mL). DMS-H11 6 (313 mg, 0.313 mmol) and a solution of Karstedt´s catalyst (~50 µL, 2 % in xylene) in anhydrous toluene (2 mL) were added which caused the solution to turn yellow after a few minutes. The solution was stirred at RT for 17 h. Activated carbon (0.5 g) was added and the suspension stirred at RT for additional 24 h. The solution was filtered through a fritted funnel packed with celite (600 mg) and eluted with n-heptane. The solvents were removed on a rotary evaporator to obtain the product 9 as a yellow oil. Yield: 388 mg (0.253 mmol, 81 %) 1_{H NMR (300 MHz, CDCl} 3) δ / ppm: 7.86-7.05 (m, 14H, ArH) 2.69 (m, 2H, CH2), 0.92 (m, 2H, CH2) 11_{B NMR (96 MHz, CDCl} 3) δ / ppm: 46.0 (s)

FT-IR ν / cm-1: 3040 (C-Haromatic), 2963 (C-Haliphatic), 1602, 1436 (C=Caromatic), 1258

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2.10. Synthesis of 4-(diphenylborane-ammoniate)phenethyl terminated PDMS

Scheme 9. Hydrosilylation reaction to 4-(diphenylborane ammoniate)phenethyl terminated PDMS 14.

The synthesis of compound 14 was a modified procedure of the literature [57]. 4-Styryl diphenylborane ammoniate 12 (182 mg, 0.639 mmol) and DMS-H11 6 (289 mg, 0.289 mmol) were dissolved in anhydrous THF (10 mL). A solution of Karstedt´s catalyst (~50 µL, 2 % in xylene) in anhydrous THF (1.5 mL) was added which lead to a yellow colourisation. The solution was stirred at RT for 17 h until there was no more conversion of Si-H groups observed in FT-IR spectroscopy. Afterwards, n-heptane (10 mL) was added and the reaction mixture cleared by filtration. The solution was washed with distilled water (3 × 20 mL), dried with magnesium sulphate, and evaporated to dryness on a rotary evaporator. The residue was washed with n-heptane (20 mL) to obtain the product 14 as a white powder.

Yield: 376 mg (0.235 mmol, 80 %) 1_{H NMR (300 MHz, CDCl} 3) δ / ppm: 7.41-6.95 (m, 14H, ArH), 3.91-3.47 (s, 3H, NH3), 2.74-2.57 (m, 2H, CH2), 1.01-0.84 (m, 2H, CH2), 0.40-(-0.20) (m, 65H, SiCH3) 29_{Si NMR (60 MHz, CDCl} 3) δ / ppm: 7.2, -20.4, -21.9, -22.0, -22.1 11_{B NMR (96 MHz, CDCl} 3) δ / ppm: -1.4 (br s)

FT-IR ν / cm-1: 3295, 3230 (N-H), 3070, 3044 (C-Haromatic), 2962 (C-Haliphatic), 1599,

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Scheme 10. Ammonium chloride cleavage to obtain 4-(diphenylborane)phenethyl terminated PDMS 9.

The synthesis of compound 9 was performed in analogy to the literature [28]. 4-(Diphenylborane-ammoniate)phenethyl terminated PDMS 14 (484 mg, 0.308 mmol) was dissolved in anhydrous Et2O (3 mL). HCl in Et2O (1M, 7 mL) was

added and the solution stirred at room temperature for 30 min. The resulting white precipitate was removed by cannula filtration. Solvent and unreacted HCl were removed in a vacuum to obtain product 9 as a yellow oil.

Yield: 403 mg (0.262 mmol, 84 %) 1_{H NMR (300 MHz, CDCl} 3) δ / ppm: 7.86-7.71 (m, 4H, ArH), 7.63-7.28 (m, 10H, ArH), 2.81-2.59 (m, 2H, CH2), 1.03-0.83 (m, 2H, CH2), 0.39-(-0.20) (m, 65H, SiCH3) 29_{Si NMR (60 MHz, CDCl} 3) δ / ppm: 7.1, -21.2, -21.3, -21.9, -22.1 11_{B NMR (96 MHz, CDCl} 3) δ / ppm: 45.9 (s)

FT-IR ν / cm-1: 3074, 3046 (C-Haromatic), 2962 (C-Haliphatic), 1603, 1497 (C=Caromatic),

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2.12. Synthesis of poly(ethenylpyridylmethyl)siloxane

Scheme 11. Hydrosilylation reaction to obtain poly(ethenylpyridylmethyl)siloxane 17.

The reaction procedure was adapted from the literature [52]. Polymethylhydrosiloxane 15 (216 mg, 3.60 mmol) was mixed with 4-vinylpyridine 16 (1.32 g, 12.6 mmol). The mixture was dissolved in anhydrous toluene (10 mL). Karstedt´s catalyst (~0.1 mL, 2 % in xylene) in anhydrous toluene (2.5 mL) was added causing a yellow colourisation of the solution. The mixture was heated to 60 °C and stirred at this temperature for 3 h. The temperature was increased to 85 °C and the solution stirred at this temperature overnight. The solvent was removed on a rotary evaporator and the resulting dark-brown solid was washed with aqueous HCl (1M, 3 × 5 mL). The product 17 was obtained as beige solid.

Yield: 324 mg (2.49 mmol, 68 %)

FT-IR ν / cm-1: 3018 (C-Haromatic), 2965 (C-Haliphatic), 2167 (Si-H), 1603, 1558, 1414

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2.13. Synthesis of poly(ethenylimidazolylmethyl)siloxane

Scheme 12. Hydrosilylation reaction to obtain poly(ethenylimidazolylmethyl)siloxane 19.

The procedure was derived from the literature [57]. Polymethylhydrosiloxane

15 (0.211 g, 3.52 mmol) was mixed with 1-vinylimidazole 18 (1.22 g, 12.9 mmol). The

mixture was dissolved in anhydrous toluene (15 mL). A solution of Karstedt´s catalyst (~0.1 mL, 2 % in xylene) in anhydrous toluene (2 mL) was added. The solution was heated to 65 °C and stirred at this temperature for 4 h. Then, the temperature was increased to 85 °C and the solution stirred at this temperature overnight. The solution was washed with distilled water (2 × 30 mL) and dried with magnesium sulphate. Afterwards, toluene was removed on a rotary evaporator to obtain the product 19 as a pale yellow solid.

Yield: 279 mg (2.86 mmol, 82 %)

FT-IR ν / cm-1: 3112 (C-Haromatic), 2965 (C-Haliphatic), 2164 (Si-H), 1649, 1511, 1493

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2.14. Synthesis of poly(ethenylpyridylmethyl) siloxane dimethyl-siloxane copolymer

Scheme 13. Hydrosilylation reaction to poly(ethenylpyridylmethyl) siloxane dimethyl-siloxane

copolymer 21.

The procedure was derived from the literature [57]. HMS-301 20 (2.32 g, 10.0 mmol) was dissolved in anhydrous toluene (40 mL). Karstedt´s catalyst (~0.1 mL, 2 % in xylene) in anhydrous toluene (2.5 mL) was added, which caused the reaction solution to turn yellow after a few seconds. 4-Vinylpyridine 16 (5.85 g, 55.7 mmol) was added over a period of 5 min. The solution was stirred at 65 °C for 3 h and afterwards at 85 °C for 14 h. The solvent was removed on a rotary evaporator to obtain a turbid brown oil, which is dissolved in n-heptane (30 mL) and washed with distilled water (4 × 10 mL). The organic phase was dried with magnesium sulphate and the solvent removed on a rotary evaporator to yield the product 21 as an orange oil.

Yield: 1.09 g (4.64 mmol, 46 %)

1_{H NMR (300 MHz, CDCl}

3) δ / ppm: 8.59-8.41 (m, 2H, ArH), 7.38-6.98 (m, 2H, ArH),

1.32-1.22 (m, 2H, CH2), 0.98-0.81 (m, 2H, CH2), 0.29-(-0.14) (m, 150H, SiCH3)

FT-IR ν / cm-1: 3073, 3029 (C-Haromatic), 2963 (C-Haliphatic), 2156 (Si-H), 1596, 1548,

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2.15. Synthesis of poly(isonicotinamidylmethyl) siloxane dimethylsiloxane copolymer

Scheme 14. Substitution reaction of AMS-162 22 with isonicotinoylchloride hydrochloride 23 to obtain

poly(isonicotinamidylmethyl)siloxane dimethylsiloxane copolymer 24.

The procedure was derived from the literature [58]. At ambient conditions AMS-162 22 (984 mg, 0.89 mmol) and triethylamine (187 mg, 1.85 mmol) were dissolved in toluene (5 mL). The solution was heated up to 50 °C. A suspension of isonicotinoylchloride hydrochloride 23 (165 mg, 0.93 mmol) in toluene (7 mL) was added over a period of 5 min. The suspension was stirred at this temperature for 30 min and afterwards refluxed for 20 h. The mixture was allowed to cool to RT before distilled water (15 mL) was added. The mixture was strongly stirred for 15 min and transferred to a separation funnel. The mixture was left to stand for phase separation for 30 min. The organic phase was washed with aqueous sodium hydroxide solution (5 w.%, 3 × 15 mL) and afterwards with saturated potassium chloride solution (2 × 15 mL). The organic phase was dried with magnesium sulphate and the toluene removed on a rotary evaporator. The product 24 was obtained as green oil. Yield: 641 mg (0.530 mmol, 60 %) 1_{H NMR (300 MHz, CDCl} 3) δ / ppm: 8.86-8.62 (m, 2H, ArH), 7.69-7.56 (m, 2H, ArH), 3.60-3.33 (m, 2H, CH2), 1.75-1.63 (m, 2H, CH2), 0.64-0.55 (m, 2H, CH2), 0.40-(-0.34) (m, 200H, SiCH3)

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3. Results and Discussion

3.1. Synthesis of diphenylborosiloxane

Scheme 15. Synthesis of diphenylborinic acid 2 via the acidic hydrolysis of 2-aminoethyl diphenyl

borinate 1.

In this approach the linkage of the boron compound to the siloxane was achieved via thermal dehydration to form a Si-O-B bond. The first step was the acidic hydrolysis of 2-aminoethyl diphenyl borinate 1 with aqueous hydrochloric acid to obtain diphenylborinic acid 2 (scheme 15). The yield of the reaction was 76 % which is lower in comparison to the literature reported 90 % [56] but still sufficiently high. The purity of the product, as well as the complete conversion, was confirmed by 1H and 11B NMR spectroscopy.

The second step was the dehydration reaction to evaporate water from the reaction mixture of diphenylborinic acid 2 and hydroxyl terminated PDMS 3 (scheme 16) to avoid the reverse reaction and ensure complete conversion to the diphenylborosiloxane 4. Since the two compounds were not miscible, they were dissolved in small amounts of xylene to obtain a homogenous solution. Vacuum was used to avoid unnecessarily high temperatures for the removal of water as the thermal stability of the boron compound is not known. Comparatively mild pressure conditions were set to avoid boiling of the siloxane at higher vacuum. Consequently, there only was a conversion of about 50 % achieved, which was

Scheme 16. Dehydration reaction of diphenylborinic acid 2 and hydroxyl terminated PDMS 3 to obtain

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Figure 15. Conversion of the dehydration reaction to diphenylborosiloxane 4 visualized by 11_{B NMR}

spectroscopy (96 MHz, CDCl3).

determined by 11_{B NMR spectroscopy, as can be seen in figure 15. This means the}

siloxane chains are on average mono-functionalized. However, to achieve crosslinking of two polymer chains it is necessary to have a di-functional chain.

Also, the solvent-free method was attempted with an emulsion of the reagents

2 and 3 which, however, leads to a lower conversion rate of about 30 %. Additionally,

the dehydration was tried in a DCM solution of the reagents by adding 4 Å molecular sieves to take up the released water. Nevertheless, the conversion rate to the dehydrated product 4 was even lower and only about 25 %. As a result, this reaction pathway was discarded. Using a higher molecular weight siloxane might be a way to improve the conversion as higher vacuum can be applied to the system without the problem of reaching the polymer´s boiling point. However, this was not investigated in the scope of this thesis.

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3.2. Synthesis of 4-(diphenylborane)phenethyl terminated PDMS by polymeric Grignard reaction

Scheme 17. Hydrosilylation reaction of 4-bromostyrene 5 and DMS-H11 6 to obtain 4-bromophenethyl

terminated PDMS 7.

The second approach to borosiloxanes was performed using a Grignard reaction to covalently connect boron atoms to the siloxane via a boron-carbon bond. The first step included the hydrosilylation reaction of 4-bromostyrene 5 to DMS-H11 6 to insert a bromine atom into the siloxane which can later serve as a starting point for the Grignard reaction. The reaction is shown in scheme 17. The reason this pathway was chosen is that the consumption of the double bond, which is necessary for the connection to the siloxane by hydrosilylation, occurs in the very first step, so there is no problem concerning self-polymerisation during the following Grignard reaction as discussed in section 1.3. As opposed to internal Si-H groups, which are investigated in section 3.4, terminal groups show good reactivity for hydrosilylation and lead to full conversion at room temperature within only a short period of time. The completeness of the reaction was observed with the disappearance of the Si-H peak in the FT-IR spectrum at 2200 cm-1. This is visualized in figure 16 in comparison to the starting material DMS-H11 6. The product was also characterized by 1H NMR spectroscopy. Minor residues of the reagent 5 were still present as its volatility is rather low making it difficult to remove it entirely by evaporation. The complexity of the spectra resulted from the lack of regioselectivity of Karstedt´s catalyst that forms both the Markovnikoff (M) and the anti-Markovnikoff (AM) products. The two products were observed in a ratio of 1:2 which was calculated according to the ratio of the aliphatic signals in the 1H NMR spectra. Therefore, there is a double set of peaks present in the spectrum than it would be the case for a regioselective reaction. This is also the case for the 29Si NMR spectra where a low-field shift of the terminal silicon atom from -6.9 ppm (6) to 6.9 ppm (AM) and 5.3 ppm (M) can be observed.

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Figure 16. FT-IR spectra of DMS-H11 6 and 4-bromophenethyl terminated PDMS 7 showing the

decreasing Si-H band at 2200 cm-1 during the hydrosilylation reaction.

The second step was the Grignard reaction of 7 with magnesium to the corresponding Grignard reagent 8. The insertion of boron was realized via a nucleophilic substitution reaction with compound 1, cleaving off the 2-aminoethanol group. The reaction is shown in scheme 18. The reaction is advantageous since the